The solar dynamo begins near the surface

The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating region of sunspot emergence appears around 30° latitude and vanishes near the equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary to theories suggesting deep origins of these phenomena, helioseismology pinpoints low-latitude torsional oscillations to the outer 5-10% of the Sun, the near-surface shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the magneto-rotational instability5,6, prominent in accretion-disk theory and observed in laboratory experiments7. Together, these two facts prompt the general question: whether the solar dynamo is possibly a near-surface instability. Here we report strong affirmative evidence in stark contrast to traditional models8 focusing on the deeper tachocline. Simple analytic estimates show that the near-surface magneto-rotational instability better explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, affecting the electromagnetic infrastructure of Earth.

Iterative methods for Navier-Stokes inverse problems

Even when the partial differential equation underlying a physical process can be evolved forward in time, the retrospective (backward in time) inverse problem often has its own challenges and applications. Direct adjoint looping (DAL) is the defacto approach for solving retrospective inverse problems, but it has not been applied to deterministic retrospective Navier-Stokes inverse problems in 2D or 3D. In this paper, we demonstrate that DAL is ill-suited for solving retrospective 2D Navier-Stokes inverse problems. Alongside DAL, we study two other iterative methods: simple backward integration (SBI) and the quasireversible method (QRM). As far as we know, our iterative SBI approach is novel, while iterative QRM has previously been used. Using these three iterative methods, we solve two retrospective inverse problems: 1D Korteweg-de Vries-Burgers (decaying nonlinear wave) and 2D Navier-Stokes (unstratified Kelvin-Helmholtz vortex). In both cases, SBI and QRM reproduce the target final states more accurately and in fewer iterations than DAL. We attribute this performance gap to additional terms present in SBI and QRM's respective backward integrations which are absent in DAL.

The photometric variability of massive stars due to gravity waves excited by core convection

Massive stars die in catastrophic explosions that seed the interstellar medium with heavy elements and produce neutron stars and black holes. Predictions of the explosion's character and the remnant mass depend on models of the star's evolutionary history. Models of massive star interiors can be empirically constrained by asteroseismic observations of gravity wave oscillations. Recent photometric observations reveal a ubiquitous red noise signal on massive main sequence stars; a hypothesized source of this noise is gravity waves driven by core convection. We present three-dimensional simulations of massive star convection extending from the star's centre to near its surface, with realistic stellar luminosities. Using these simulations, we predict the photometric variability due to convectively driven gravity waves at the surfaces of massive stars, and find that gravity waves produce photometric variability of a lower amplitude and lower characteristic frequency than the observed red noise. We infer that the photometric signal of gravity waves excited by core convection is below the noise limit of current observations, and thus the red noise must be generated by an alternative process.

Improved phase-field models of melting and dissolution in multi-component flows

We develop and analyse the first second-order phase-field model to combine melting and dissolution in multi-component flows. This provides a simple and accurate way to simulate challenging phase-change problems in existing codes. Phase-field models simplify computation by describing separate regions using a smoothed phase field. The phase field eliminates the need for complicated discretizations that track the moving phase boundary. However, standard phase-field models are only first-order accurate. They often incur an error proportional to the thickness of the diffuse interface. We eliminate this dominant error by developing a general framework for asymptotic analysis of diffuse-interface methods in arbitrary geometries. With this framework, we can consistently unify previous second-order phase-field models of melting and dissolution and the volume-penalty method for fluid-solid interaction. We finally validate second-order convergence of our model in two comprehensive benchmark problems using the open-source spectral code Dedalus.

The magnetorotational instability prefers three dimensions

The magnetorotational instability (MRI) occurs when a weak magnetic field destabilizes a rotating, electrically conducting fluid with inwardly increasing angular velocity. The MRI is essential to astrophysical disc theory where the shear is typically Keplerian. Internal shear layers in stars may also be MRI-unstable, and they take a wide range of profiles, including near-critical. We show that the fastest growing modes of an ideal magnetofluid are three-dimensional provided the shear rate, S, is near the two-dimensional onset value, S _c . For a Keplerian shear, three-dimensional modes are unstable above S ≈ 0.10S _c , and dominate the two-dimensional modes until S ≈ 2.05S _c . These three-dimensional modes dominate for shear profiles relevant to stars and at magnetic Prandtl numbers relevant to liquid-metal laboratory experiments. Significant numbers of rapidly growing three-dimensional modes remainy well past 2.05S _c . These finding are significant in three ways. First, weakly nonlinear theory suggests that the MRI saturates by pushing the shear rate to its critical value. This can happen for systems, such as stars and laboratory experiments, that can rearrange their angular velocity profiles. Second, the non-normal character and large transient growth of MRI modes should be important whenever three-dimensionality exists. Finally, three-dimensional growth suggests direct dynamo action driven from the linear instability.

Anomalous Chained Turbulence in Actively Driven Flows on Spheres

Recent experiments demonstrate the importance of substrate curvature for actively forced fluid dynamics. Yet, the covariant formulation and analysis of continuum models for nonequilibrium flows on curved surfaces still poses theoretical challenges. Here, we introduce and study a generalized covariant Navier-Stokes model for fluid flows driven by active stresses in nonplanar geometries. The analytical tractability of the theory is demonstrated through exact stationary solutions for the case of a spherical bubble geometry. Direct numerical simulations reveal a curvature-induced transition from a burst phase to an anomalous turbulent phase that differs distinctly from externally forced classical 2D Kolmogorov turbulence. This new type of active turbulence is characterized by the self-assembly of finite-size vortices into linked chains of antiferromagnetic order, which percolate through the entire fluid domain, forming an active dynamic network. The coherent motion of the vortex chain network provides an efficient mechanism for upward energy transfer from smaller to larger scales, presenting an alternative to the conventional energy cascade in classical 2D turbulence.

Rolling resistance of shallow granular deformation

Experiments are conducted to measure the resistance experienced by light cylinders rolling over flat beds of granular media. Sand and glass spheres are used for the beds. The trajectories of the rolling cylinders are determined through optical tracking, and velocity and acceleration data are inferred through fits to these trajectories. The rolling resistance is dominated by a velocity-independent component, but a velocity-dependent drag exceeding the expected strength of air drag is also observed. The results are compared to a theoretical model based on a cohesionless Mohr–Coulomb rheology for a granular medium in the presence of gravity. The model idealizes the flow pattern underneath the rolling cylinder as a plastically deforming zone in front of a rigidly rotating plug attached to the cylinder, as proposed previously for cylinders rolling on perfectly cohesive plastic media. The leading-order, rate-independent rolling resistance observed experimentally is well reproduced by the model predictions.

Numerical simulations of internal wave generation by convection in water

Water's density maximum at 4°C makes it well suited to study internal gravity wave excitation by convection: an increasing temperature profile is unstable to convection below 4°C, but stably stratified above 4°C. We present numerical simulations of a waterlike fluid near its density maximum in a two-dimensional domain. We successfully model the damping of waves in the simulations using linear theory, provided we do not take the weak damping limit typically used in the literature. To isolate the physical mechanism exciting internal waves, we use the spectral code dedalus to run several simplified model simulations of our more detailed simulation. We use data from the full simulation as source terms in two simplified models of internal-wave excitation by convection: bulk excitation by convective Reynolds stresses, and interface forcing via the mechanical oscillator effect. We find excellent agreement between the waves generated in the full simulation and the simplified simulation implementing the bulk excitation mechanism. The interface forcing simulations overexcite high-frequency waves because they assume the excitation is by the "impulsive" penetration of plumes, which spreads energy to high frequencies. However, we find that the real excitation is instead by the "sweeping" motion of plumes parallel to the interface. Our results imply that the bulk excitation mechanism is a very accurate heuristic for internal-wave generation by convection.

Mental models of line drawings

Mental models are internal representations of world structure, used to accomplish cognitive tasks. I postulate specific representations (of objects and images) and associated context (of world and view) for mental models of line drawings. I then analyze the representations and context to predict specific perceptual modes, including the relative strengths of these modes. The predicted modes are supported by a well-known example [from Rock, 1983 The Logic of Perception (Cambridge, MA: MIT Press)] where object perception changes with image orientation.

Power and effect size: research considerations for the clinical nurse specialist

Frequently, a goal for a Clinical Nurse Specialist (CNS) is to determine whether research results are clinically meaningful and appropriate for adoption into practice. Often, the evaluation of research involves an examination of the efficacy and cost-effectiveness of the intervention under study. Attention to two concepts from research methodology--power and effect size--can help the CNS evaluate research findings and design studies to produce clinically meaningful results.

Predictors of referral to cardiac rehabilitation and cardiac exercise self-efficacy

Cardiac rehabilitation (CR) has known benefits after myocardial infarction (MI) or coronary artery bypass surgery (CABG). Yet, only a small percentage of patients are referred for outpatient CR after hospital discharge. This study investigates patient characteristics related to referral to CR and cardiac exercise self-efficacy, a salient predictor of health behavior change and maintenance. Two hundred nineteen patients enrolled in the study. Of the 185 patients who were CR candidates, 74 were referred to CR. Logistic regression analysis was used to identify variables related to CR referral. Results indicate that patient characteristics of having had fewer MIs or CABGs, having attended CR in the past, and being less physically active during leisure time are related to an increased likelihood of being referred to CR. Multiple regression analysis indicates that leisure physical activity is a predictor of cardiac exercise self-efficacy. Implications for nurses who recruit patients for CR are discussed.

Refinement of the Habitual Physical Activity Index for use with American adults

The relationship between physical activity and health outcomes makes the assessment of physical activity important to many health professionals. Numerous physical activity questionnaires exist but none is tailored to the assessment of physical activity in cardiac patients. Ideally, a questionnaire should be appropriate for both healthy individuals and those with cardiac disease to allow comparison of epidemiologic and intervention research. The purpose of this research is to adapt and validate an existing physical activity questionnaire for use with healthy individuals and those with coronary artery disease. The Habitual Physical Activity Index (HPAI) is an easy to administer questionnaire developed in the Netherlands by Baecke, Burema, and Frijters (1982). The factorial validity and alpha internal consistency of the HPAI were examined to evaluate its potential for use in assessing physical activity of hospitalized cardiac patients in the U.S. The sample was 213 cardiac patients. A factor analysis produced a meaningful two-factor solution that differed from the original Dutch results. This suggested the HPAI should be modified for use with American adults with cardiac disease. New items were generated and added to the HPAI. The modified HPAI was tested using classical testing theory and generalizability theory. Stability estimates are high and relative generalizability acceptable. However, the absolute generalizability estimates indicate that the addition of new items to the HPAI could improve its use for absolute decision making.

Beyond classical reliability: using generalizability theory to assess dependability

Generalizability theory (GT), an approach to estimating measurement dependability, offers researchers the ability to assess comprehensively various sources of measurement error simultaneously. Application of GT identifies the conditions that contribute most substantially to error and offers remedies to enhance dependability. A review of GT is presented and a practical application discussed. Researchers who develop instruments or plan studies using observation or questionnaires will appreciate the advantages offered by GT.

Molecular systematics of tanagers (Thraupinae): evolution and biogeography of a diverse radiation of neotropical birds

The tanagers (Passeriformes: Emberizidae: Thraupinae) are a diverse group of mostly Neotropical birds with a wide range of feeding morphologies, behaviors, plumage patterns and colors, and habitat preferences. Phylogenetic relationships of genera in this lineage were investigated using cytochrome b sequence data. This study indicates that the genera Euphonia and Chlorophonia (traditionally considered part of Thraupinae) do not form a monophyletic group with the other tanagers. Within the rest of Thraupinae, several monophyletic groups are identified that agree with traditional sequential taxonomies. Other monophyletic groups provide novel interpretations of biogeographic patterns and morphological evolution within tanagers. In several lineages, plumage patterns and colors persist despite dramatic changes in bill morphology. Phylogenetic structure and estimated timings of divergence events indicate that tanagers probably originated on Caribbean islands and later diversified throughout Central and South America during the mid-Tertiary.

Counseling for physical activity: what primary-care physicians should know

In Connecticut, 22.1% of the adult population report no physical activity other than activities of daily living. Primary care physicians can play a vital role in the flight against a sedentary lifestyle. As outlined in Healthy People 2000 and the Surgeon General's Report on Physical Activity and Health, the primary-care physician's role in the assessment and advising of physical activity is critical. The primary-care physicians that responded to the Connecticut survey appear to be doing an excellent job in delivering this important message. Recommendations made by the primary-care physician can be a great source of motivation for the patient. The primary-care physician's initial goal should be to get the patient active on a daily basis and than progress to more health-based fitness goals. The patient needs to receive instruction on goal setting, safe exercise tips, and adherence strategies. The primary-care physician's low-cost intervention of advising physical activity can help prevent medical problems in the future. The most current recommendation on physical activity states that, "All U.S. adults should accumulate 30 minutes or more of continuous or intermittent amounts of moderate-intensity physical activity on most, preferably all, days of the week." This latest recommendation offers primary-care physicians a broader range of activities to suggest to their patients. An overwhelming number of research studies indicate that physical activity is a powerful weapon in combating a host of lifestyle-related health problems.

A new recommendation for physical activity as a means of health promotion

The Centers for Disease Control and Prevention and the American College of Sports Medicine have issued a recommendation for physical activity to achieve health promotion and disease prevention. The recommendation signals a shift in the exercise-fitness paradigm to emphasize the relationship between physical activity and health. A distinction is made between scheduled vigorous exercise and Intermittent moderate physical activity. Moderate physical activity, which can include occupational, household, and recreational activity, is recommended for all Americans. Contrary to previous beliefs, one no longer needs to adhere to a rigid schedule of vigorous exercise to achieve health benefit. Through the promotion of physical activity, primary care practitioners have an opportunity to significantly impact the nation's public health. Strategies to help clients achieve the desired activity levels are needed. Careful assessments and creative strategies will help clients meet the physical activity objective.

Cross-protective immunity and the serological classification system for Bacteroides nodosus

The relationship between the serological classification system for serogroup B and for serogroup H of Bacteroides nodosus and cross-protection between subgroups within these serogroups was examined. Protection against ovine footrot following vaccination was achieved against other subgroup strains provided sufficient cross-reactive antibody was induced by shared pilus antigens. Within serogroup B, better cross-protection against one subgroup was obtained with a pili vaccine than a whole cell vaccine which correlated with higher pilus antibody titres induced by the former. For serogroup H, a lack of cross-protection and serological reactivity between subgroups was demonstrated, which indicates that the prototype strain of subgroup H2 should be designated a new serogroup.